The present invention relates to a method of laying out a photomask, and more particularly, to a method of fabricating an assist feature in a photomask.
In recent, as an integration degree of a semiconductor device has been rapidly increased, a pitch of a pattern formed on a wafer has been continuously narrowed. As the result, a limitation in a photography process for fabricating the pattern has become more serious. Accordingly, to form a pattern requiring high resolution, a laser with a short wavelength and a projection lens having a high numerical aperture have been used. However, there occurs a problem that a margin in a depth of focus is lowered as the numerical aperture is increased. Therefore, various methods for solving the problem are used and one of them is a method of inserting an assist feature in a periphery of a main pattern. Particularly, the insertion of the assist feature is mostly necessarily required to form a pattern where an isolated pattern, independently disposed spaced apart from the main pattern at a considerable distance, and a dense pattern, arranged close to other patterns, coexist.
FIGS. 1 and 2 are layouts illustrating a conventional method of fabricating an assist feature in a photomask. Also, FIGS. 3 and 4 are illustrations showing the pattern formed on a wafer using the photomask having the assist feature fabricated by the conventional method. First, as illustrated in FIG. 1, main patterns 110 are placed on a light transmitting substrate 100. The main patterns 110 are, for example, patterns for fabricating a contact hole, and arranged in an irregular arrangement including a dense pattern and an isolated pattern. Next, as illustrated in FIG. 2, assist patterns 120 are placed in a periphery of the main patterns 110 according to a photomask fabrication rule. The photomask fabrication rule can be made by an exposure simulation. Successively, an optical proximity correction is implemented and a practical photomask is then fabricated on the basis of the fabricated layout.
However, in this conventional method, the fabrication rule of situating the assist pattern 120 is influenced by the arrangement of the main patterns 110, and a portion which clashes with the mask fabrication rule is generated or a portion in which the assist feature cannot be inserted by the mask fabrication rule is generated in a region in which adjacent main patterns 110 are arranged irregularly as indicated by “A” in FIG. 2. When implementing an actual exposure using the photomask fabricated as such, a pattern defect can result as indicated by “B” in FIG. 3 and “C” in FIG. 4.